A phase object, such as a cell object, is transparent to light. However, the light may have a phase variation due to an internal structure of the phase object. Therefore, to obtain the internal structure of the phase object, the phase variation of the light is required to be detected for converting into image contrast. Namely, the image of the phase object is formed as the light passing through the phase object to generate bright and dark contrast due to an optical path difference.
Regarding a design of a microscope, the depth of field is usually quite short due to a large numerical aperture (NA). Moreover, although a confocal microscope may have effect to extend field depth, it takes long time for multiple imaging processes at different depths, causing low speed. Although the depth of field can be extended by reducing the aperture, light flux would decrease.
A present microscope system for phase object approximately has a phase contrast structure, a transmissive differential interference contrast (DIC) structure, or a reflective DIC structure. An image of a conventional microscope system for phase object maintains a high resolution only within a range of the depth of field, and the resolution of the image is severely decreased beyond the range of depth of field. In order to extend the depth of field, the conventional structure is added with a phase device to code a wavefront through a non-axial symmetric method.
Since the structure of phase device is non-axial symmetric, it is difficult to process the phase device, and the geometric structure in fabrication has a low accuracy. Besides, the generated point spread function (PSF) being generated is non-axial symmetric, which may increase difficulty in image restoration.